Various types of microphone systems have been used in various applications through the years. Microphones in these systems typically receive acoustic energy and convert this acoustic energy into an electrical voltage. This voltage can be further processed by other applications or for other purposes. For example, in a hearing aid system the microphone may receive acoustic energy, and convert the acoustic energy to an electrical voltage. The voltage may be amplified or otherwise processed by an amplifier, or by other signal processing electronics circuitry, and then presented by a receiver as acoustic energy to a user or wearer of the hearing aid. To take another specific example, microphone systems in cellular phones typically receive sound energy, convert this energy into a voltage, and then this voltage can be further processed for use by other applications. Microphones are used in other applications and in other devices as well.
One type of microphone that is known and used is a condenser microphone. The condenser microphone operates as a variable capacitor whose value is modulated by the pressure of an incoming sound wave. Viewed as a capacitor having two plates, one of the capacitor plates is static, while the other one is mobile (i.e., the diaphragm of the microphone). The sound wave changes the distance between the plates and thereby the capacitance C of the capacitor.
A microelectromechanical system (MEMS) microphone is a variant of the condenser microphone and is formed by using silicon micro-fabrication techniques. Compared to the conventional condenser microphone, it has several advantages such as a reduced size, a lower temperature coefficient, and a higher immunity to mechanical shocks. In addition, the MEMS microphone takes advantage of a lithography process which is very suitable for mass production of devices.
One of the most common methods to obtain useful electrical signals from such microphones is to maintain a constant charge Q on the capacitor C. The voltage across the capacitor will change inversely proportionally to the incoming sound wave pressure according to the equation V=Q/C, consequently dV=−VdC/C.
Unfortunately, the sensitivity (indicated by dV) depends upon the voltage V and it is difficult to obtain a high voltage V in a standard low voltage CMOS process. This has led to either microphones with inadequate sensitivities for many applications or microphones with high prices. As a result, there has been dissatisfaction with previous approaches.
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